Language packs in SharePoint are great, when you need to provision team sites in different languages. But remember, that Microsoft publishes language packs for SharePoint Services and for Office SharePoint Server. To have a language pack in your MOSS-installation, you have to install the language pack for SharePoint Services and the language pack for Office SharePoint Server. Keep this in mind, when you need to install service packs, because when installing a service pack for Office SharePoint Server, you also need to install the service pack for your language packs. Here you need to install the service pack for the SharePoint Services language pack AND the service pack for the Office SharePoint Server language pack. After installation don’t forget to run the Configuration Wizard.
When your environment runs in Windows Server 2008, have a look in Harold van de Kamp’s Blog, how to install the language pack service packs in this environment.